The Flavian Amphitheater, better known as the Colosseum, stands as one of the most spectacular architectural monuments of the ancient world. Built in the first century A.D., it’s largely remembered as the site of blood-sport entertainment involving gladiators, wild animals and more. But as one of ancient Rome’s best surviving and most iconic structures, it remains an enduring monument to one of the most influential dynasties of the Roman Empire—and a marvel of architecture and engineering.

After Vespasian became Roman Emperor in 69 A.D., his Flavian Dynasty— which included his sons, Titus and Domitian—launched a vast building program to restore Rome, which had been ravaged by fire, plague and civil war. During the Flavian Dynasty’s 27-year reign, it renovated buildings, statues and monuments throughout the city. In 70 A.D., Vespasian ordered the construction of the new amphitheater in the city center, funded with the spoils from the Roman siege of Jerusalem during the First Jewish-Roman War. The Colosseum, dedicated 10 years later, served as a dramatic political symbol of the city’s resurgence.

It was also an innovative architectural and engineering wonder, the largest and most complex permanent amphitheater of the ancient world. Made primarily of concrete, 3.5 million cubic feet of travertine, and similar amounts of marble, stone and timber, the Colosseum rose to 157 feet (roughly the height of a 15-story building), with a capacity for an estimated 50,000 to 80,000 people.

“The Colosseum…was part of an entire complex of buildings that Vespasian and his sons were building throughout Rome as part of a bigger program to erase [their predecessor] Nero’s mark on the city—and to champion their own achievements,” says Nathan Elkins, deputy director of the American Numismatic Society and author of Monument to Dynasty and Death: The Story of Rome’s Colosseum and the Emperors Who Built It. At its dedication, Titus presided over 100 days of games, which included gladiatorial combat and animal entertainment.

Builders situated the Colosseum on the site of Nero’s estate, Domus Aurea, which featured an artificial lake and a 98-foot bronze statue of himself, the Colossus of Nero. They filled in the lake to build the Colosseum, which took its name from its proximity to the statue. When Nero committed suicide in 68 A.D., Vespasian, one of his generals, rose to power after a civil war.

Building the Colosseum offered a clever way for the Flavian Dynasty to satisfy the dictates of Roman society’s rigid social hierarchy, says Elkins. Nero had made his estate accessible to all, but the senators didn’t like the access he was allowing for common people in the center of the city. “But by building this massive amphitheater, [Vespasian and his sons] keep this area a place for public enjoyment with games and also use it to reinforce Roman social order with hierarchical seating,” Elkins says.

In the Colosseum, social status, wealth and gender determined where people sat. The best seats, closest to the arena, were reserved for the Emperor and senatorial nobility. Above them sat the Equestrian order, former cavalry members who had become established merchants, artisans and bureaucrats. Above them, in the nosebleed seats, sat the other 95 percent of Rome’s population: women, foreigners, and poor and enslaved Romans.

To facilitate the orderly flow of people throughout the structure, builders gave the Colosseum four entrances for the political and religious leaders and 76 for the ordinary citizens. Corridors separated social groups from one another, barring spectators from moving freely within the structure. But while seating wasn’t equal for all Roman citizens, the Colosseum’s elliptical architecture gave everyone visibility to the action on the arena floor.

Throughout ancient Rome, most amphitheaters were built as temporary structures made of wood for gladiatorial games and other amusements. The earliest known permanent amphitheater—a stone structure built for some 20,000 spectators—dates to 80 B.C.E. in Pompeii.

The architect of the Colosseum remains unknown. “The Colosseum’s form is deeply connected with earlier structures used to entertain crowds, such as Greek theaters,” wrote art historian Peter Louis Bonfitto in his book World Architecture and Society: From Stonehenge to One World Trade Center. Its grand design employs an impressive series of columns, arches and barrel vaults.

The Colosseum’s greatest innovation, says Elkins, was its use of concrete. “The concrete construction is really what allows the Colosseum to be built,” he said. “It was probably the most widespread use of engineering and construction with concrete in that period of time.”

According to contemporary engineers, the Colosseum remains standing after 2,000 years because of its solid concrete foundation. Building in a wetland area near the Tiber River, with poor soil conditions, forced builders to dig a deep and strong foundation to stabilize the structure, according to Engineering Rome, a University of Washington program that explores Roman and Italian engineering.

It featured other innovations as well, including a sophisticated drainage system used to siphon off water used to stage mock sea battles in the arena. Sailors were employed to operate an overhead retractable awning, which could be rolled out to protect spectators from rain or Rome’s blistering heat. The complex network of chambers and tunnels beneath the arena floor, called the hypogeum, housed props, scenery and participants when not in action. And the amphitheater’s ingenious system of trap doors, pulleys and lifts facilitated dramatic entrances for scenery and combatants alike, allowing even elephants to appear as if from nowhere.

While it’s unknown what it cost to build the Colosseum in antiquity, many scholars believe the Colosseum was partly financed with the booty taken by Roman soldiers during the empire’s raid of the Jerusalem Temple during the First Roman-Jewish War that ended in 70 A.D. An inscription at the Colosseum reads: The Emperor Titus Caesar Vespasian Augustus ordered the new amphitheater to be made from the (proceeds from the sale of the) booty.

For generations, the conventional wisdom has been that the labor to build the Colosseum was carried out by 100,000 Jewish slaves captured during the Siege of Jerusalem, but Elkins isn’t entirely convinced. “It’s the kind of thing Romans might do to add insult to injury,” Elkins said. “You not only sell them into slavery, but then you make them build something that is financed by the destruction of their temple.”

But the assertion, he says, is unsupported by an ancient source. “It came from a 20th-century archaeologist, and it has been repeated over and over again. A significant amount of slaves would have been used, but we don’t know 100 percent where those slaves came from.”

Beyond functioning as a window into ancient Rome and its social structure, the Colosseum is also the father of all modern outdoor sports stadiums. The Colosseum’s use of arches to support the structure, the elliptical shape and the organizational system used to control the entry and exit of fans based on the location of their seats are staples of most modern stadiums.